It is known that certain sulfenamides having the characteristic nucleus ##STR1## inhibit premature vulcanization of rubber. For example, see U.S. Pat. Nos. 3,546,185; 3,686,169; 3,705,135; 3,725,361; 3,732,271; 3,775,428; 3,780,001 and 3,862,051. The present invention concerns a different class of sulfenamides in which the aforesaid characteristic nucleus is part of a heterocyclic ring in which the dangling valence of nitrogen is satisfied by another nitrogen atom.